Greenwich Mean Time (GMT) was adopted as the world’s time standard at the Washington Meridian Conference in 1884. This conference also established Universal Time, from which the international 24-hour time-zone system grew. This is why all time zones refer back to GMT on the prime meridian. The prime meridian at Greenwich, in the United Kingdom, has served as the reference line for GMT since the late 19th century.
The prime meridian was defined by the position of the large “Transit Circle” telescope in the Observatory’s Meridian Building in 1884. The cross-hairs in the Transit Circle’s eyepiece precisely defined longitude 0° for the world. The Earth’s crust is moving very slightly all the time so the prime meridian’s exact position is now moving very slightly too. However, the original reference for the world’s prime meridian is still the Airy Transit Circle in the Royal Observatory in the United Kingdom.
GMT was at first calculated by the 24-hour clock starting at noon. However, since 1925, the day of GMT starts at midnight. GMT was later renamed to Universal Time, or UT. It is also known as UT0. It becomes UT1 when it is corrected for the irregular movements of the terrestrial poles, also known as the Chandler wobble. The Earth’s poles do not spin perfectly in a straight line.